Method for lignin separation from black liquor involving multiple acidification steps

ABSTRACT

The method is for separation of lignin from original black liquor (BL IN ) and has the following phases in sequence. A first precipitation phase (PR 1 ) for precipitation of lignin by a first acidification of the original black liquor by adding a first acid or mixture of acids (G 1a ) followed by a first storage phase wherein precipitated lignin particles are allowed to increase in size, followed by a second precipitation phase (PR 2 ) for precipitation of lignin by a further acidification of the original black liquor by adding a second acid or mixture of acids (G 1b ). By retaining the acidified black liquor for at least 25 minutes in the storage phase are the precipitated lignin particles allowed to grow in size and the filterability is improved considerably.

PRIOR APPLICATION

This application is a U.S. national phase application that is based onand claims priority from International Application No.PCT/SE2011/051360, filed 11 Nov. 2011.

TECHNICAL FIELD

This invention relates to a method for lignin separation from spentcooking liquor, called original black liquor, using a precipitationprocess.

BACKGROUND

The advantages with lignin separation from black liquor is alreadydescribed in WO 2006/031175 and WO02006/038863. These patents disclosethe novel process LignoBoost™ that is now sold by Metso, and wherein WO2006/031175 discloses the basic two stage acidic wash process andWO2006/038863 disclose an improvement of the process where sulphate orsulphate ions are added to the process.

An important aspect of the process is that the required charge ofchemicals for the acidification may be high. If this is the case thecost of fresh chemicals is a large part of the operational cost and thecommercial viability of the process is lower. These problems could bereduced, if the process is optimized for minimum requirement for chargesof fresh chemicals, making the lignin product commercially sound.Acidifiers in form of mill generated waste flows are thus preferable asit may solve a waste disposal problem and lessen environmental impact.As the precipitation of lignin requires acidification of alkaline blackliquor flows, much of the total amount of acidifier is used to decreasethe pH level down to the point of where lignin starts to precipitate.The first phase reaching this pH level typically reduce the pH levelfrom about pH 13 in the original black liquor down to a pH level about11.5, and normally do not involve any nucleation of lignin particles.The amount of acidifier needed is nevertheless relatively high for thisfirst phase as the pH follows a logarithmic scale, and any followingadditional lowering of pH from 11.5 requires far less acidifier for thesame order of lowered absolute pH value.

The Lignoboost process produce a lignin product which if used as fuel isclassified as a “green” fuel as being based upon recovered fuel. Theidea with classification of “green” fuels is based upon the concept notto increase the carbon dioxide footprint, i.e. the emissions, by burningfossil fuels. The most promising acids for this process is carbondioxide for at least initial precipitation of the lignin, and then usingsulfuric acid (H₂SO₄) for washing and leaching out metals from thelignin. The sulfuric acid could be added as a fresh sulfuric acid from achemical supplier, or as preferred using so called “spent acid” from achlorine dioxide generator often used at a pulp mill. The latter usageof this spent acid already at hand in most mill sites further emphasizethat the lignin product is considered as a “green” fuel.

Another problem with the process disclosed in WO 2006/031175 is thatthere may be a disposal problem with the strongly odorous H₂S gases thatare emitted from the reslurrying tank and bled out from the process, andit is suggested that these hydrogen sulfides could be added to thepulping process in order to increase sulfidity and possibly increase theyield in the pulping process. However, such rerouting of the stronglyodorous H₂S gases to another part of the pulp mill introduces risks foremissions of these gases during transport and storage. It is far betterto use these gases at the location or process producing these gases.

As the chemical constitution of the original black liquor may changeduring operation, typically due to changes in the pulping process as ofchanges in wood material used or cooking conditions, the firstprecipitation process for precipitating lignin particles from theoriginal black liquor may need adaption to the present conditions. Asdiffering requirements apply for the first precipitation phase, wheremainly lowering of pH is the objective, and the second precipitationphase, where lignin starts to precipitate it will be difficult to designa system that meets both these requirements.

As the precipitated lignin needs to be separated from the acidifiedblack liquor slurry, which still is kept alkaline at a pH level above 7,it is important that the filterability of the precipitated lignin ishigh. If the filterability is improved could smaller separationequipment be used, and less investments is needed.

SUMMARY OF THE INVENTION

The invention is based upon the surprising finding that theprecipitation process should be divided into at least two distinctivephases, each adapted for the present phase in the precipitation phaseand having its own supply of acidifier, and that the initially acidifiedblack liquor volume after the first phase should be kept in a storagevessel allowing the acidified black liquor to mature before starting thefinal acidification. By such retention of the acidified black liquor ina storage vessel could the filterability be improved almost trefoldcompared to an acidification without such storage.

Thus, the invention is related to a method for separation of lignin fromoriginal black liquor having a first pH value, comprising the followingphases in sequence:

a first precipitation phase wherein a first acidifier charge is added tothe original black liquor in order to decrease the pH value of theoriginal black liquor to a second pH level whereby less than 10% of thetotal lignin content is precipitated as nucleus particles, said secondpH level being at least 1 pH unit below that of the first pH value,

a first storage phase (ST) wherein the original black liquor is kept ator below the second pH level for a retention time of at least 25 minutesduring which storage phase the precipitated lignin particles increase insize thus increasing the filterability of the precipitated lignin

a second precipitation phase wherein a second acidifier charge is addedto the acidified original black liquor from the first precipitationphase in order to decrease the pH value to a third pH level whereby morethan 20% of the total lignin content is additionally precipitated andpreferably as growth of nucleus lignin particles formed after the firststorage phase, said third pH level being at least 1 pH units below thatof the second pH value,

followed by a separation phase wherein the precipitated lignin isseparated from the remaining liquid phase of the acidified originalblack liquor.

By this method could precipitation be adapted for each individual phasewith its individual charge of acidifier, charged in order to meet theobjective of each phase.

Preferably is 50-80% of the total lignin content in original blackliquor (BL_(IN)) precipitated in total after the second precipitationphase, and that the pH level of the acidified original black liquor isstill alkaline, i.e. has a pH level above 7.0 and preferably about 10,after the second precipitation phase. By this embodiment could a part ofthe total lignin content, typically about 70%, be extracted from theoriginal black liquor, still keeping a part of the heat value of thetreated black liquor for any subsequent combustion in a recovery boiler,and the remaining liquid part of the original black liquor could bemixed back into the major part of the original black liquor not causingany problems associated with mixing of acidic waste flows to blackliquor.

According to one preferred embodiment is also at least one of the firstor second acidifier charges comprising acidifying gas. I.e. liquidacidifier could also be used, but acidic waste gases are often availableat a pulp mill and a potential environmental pollution if not destructedin expensive waste gas cleaning systems. It is thus preferable to usethese gases as acidifiers in the inventive method. Preferably theacidifying gas is rich in carbon dioxide, and may have its origin fromflue gases vented from a lime kiln which naturally contains largeamounts of carbon dioxide.

As the inventive method includes at least two distinct phases usingacidifying gas charged could at least a part of the flow path of thefirst acidifying gas led trough the first precipitation phase have arandom flow path constantly changing flow direction at no straight flowpath longer than 5 centimeter, preferably less than 1 centimeter, saidflow path created by random packing of filling bodies in said flow path.Such a routing of the gases trough the flow of black liquor increase thedissolving capacity of the acidifying gas and hence obtain a same pH insaid phase with less charge of acidifier gas or lower pH with similarcharge. The filling bodies used could preferably be of a type similar toRachig-rings normally used in gas contacting columns or filters, orother shape of irregular filling bodies.

In a further embodiment could also at least a part of the flow path ofthe original black liquor from the first precipitation phase led troughthe second precipitation phase have an open flow path allowing astraight flow path longer than 5 centimeter, with flow restrictionsallowing precipitated lignin to move with the flow of the black liquorwith a flow deflection of the precipitated lignin being less than 80degrees in relation to the general flow direction of the black liquortrough the second precipitation phase, hence allowing any precipitatedlignin particles flow with at least one flow vector being parallel tothe general flow. By this design could be avoided that precipitatedlignin may block the flow path of the black liquor and totally stop theprocess.

In yet a further embodiment of the inventive method using acidifier gasis the original black liquor flowing downwards in the firstprecipitation phase wherein a first acidifier gas is led countercurrentto flow of original black liquor. This embodiment may enable longerretention time of the acidifier in the flow of black liquor, andincrease the dissolving capacity of the acidifying gas.

As an alternative embodiment of the inventive method is the originalblack liquor flowing upwards in the first precipitation phase wherein afirst acidifier gas is led concurrent with flow of original blackliquor. This may be preferable if a lower concentration of acidifier gasis needed in the position where precipitation of lignin nucleus particlemay start, as high concentration of acidifier gas may result inexcessive formation of small nucleus particles instead of ligninparticle growth.

Most of the acidifier needed for acidification and precipitation of thelignin from the black liquor could be obtained from flue gases ventedfrom a lime kiln at the mill site. Typically the content of carbondioxide in these flues gases is well above 25%. By using these fluegases for acidification and precipitation would emissions from the limekiln in aspects of carbon dioxide be reduced significantly, and no freshcarbon dioxide needs to be added to the Lignoboost process. Only byusing the flue gases from the lime kiln could the pH of the black liquorbe lowered by 1.5 to 2.5 units, i.e. from an original pH level above pH13 down to a pH level in the order of 11.5, thus only initiating asmaller first precipitate fraction of lignin from the original blackliquor mostly containing small lignin nucleus particles

In yet a preferred embodiment of the inventive method is at least a partof the flue gases vented from the lime kiln first used for dewateringthe lignin cake before being used as acidifier in the firstprecipitation phase. This improves the dewatering of the lignin productas well as takes care of any environmental problems with dust emissionsfrom the dewatering phase. The dust would then be brought into theprecipitation phase and collected in the lignin product precipitated.

In a further preferred embodiment of the invention are also furthercarbon dioxide and H₂S gases emitted from second acidification phase inthe Lignoboost process re circulated and mixed with the original blackliquor in the first precipitation phase. By using this re-circulationcould almost the entire need for added acidifier in the precipitationphase be fulfilled by using only lime kiln flue gases and internal gasesfrom the process. If the Lignoboost process is implemented toprecipitate lignin from a semi-evaporated original black liquor having aconcentration of solids of about 42%, could as much as 9.6 ton of ligninper hour be precipitated from a black liquor flow of about 103m³/h.

The H₂S gases that are emitted from the reslurrying tank in theLignoboost process contain a large amount of residual carbon dioxide,CO₂. By re-circulating this H₂S and CO₂ rich gas back to the firstacidification phase a corresponding reduction of addition of the freshcarbon dioxide is obtained. Only by using the flue gases from the limekiln in a first phase could the pH of the black liquor be lowered by 1.5to 2.5 units, i.e. from an original pH level above pH 13 down to a pHlevel in the order of 11.5, thus initiating a first precipitate fractionof lignin from the original black liquor, and re circulation of the H₂Sgases emitted from second acidification phase to the precipitation phasecould lower the pH further down from 11.5 down to a pH level in. theorder of 11.2, thus initiating a second larger precipitate fraction oflignin from the original black liquor.

As indicated above could the precipitation stage be implemented in firstand second phases which either could be implemented in one and the samevessel or in two separate vessels. When the precipitation stagecomprises two separate precipitation phases, treating the original blackliquor in series, could at least a part of the gases rich in carbondioxide and having its origin from flue gases vented from a lime kiln beadded to the first phase of the first precipitation stage. As the limekiln flue gases comes in great volumes could this absorption process forthe carbon dioxide content be optimized for these large gas volumes.

If the precipitation stage comprises two separate precipitation phases,then the waste gases emitted from the second acidification stage couldbe re circulated and mixed with the original black liquor in the secondphase of the first precipitation stage.

The carbon dioxide formed in the reslurrying tank, originates from thesulphides and carbonates content in the lignin cake. These compoundsreact with the acidifier and forms carbon dioxide (CO₂) and hydrogensulfide (H₂S), according to:CO₃ ²⁻+2H⁺<−>CO₂+H₂OHCO₃ ⁻+H⁺<−>CO₂+H₂OS²⁻+2H⁺<−>H₂SHS⁻+H⁺<−>H₂S

The formation of carbon dioxide in this process enables a further sourcefor carbon dioxide needed for the first acidification phase, and thehydrogen sulfide is also a net contributor to the acidification as thepK_(a) value of hydrogen sulfide is 6.89.

In a further preferred embodiment of the inventive method are the fluegases vented from a lime kiln first used for dewatering the lignin cakeor lignin product in at least one of the first, second and/or thirddewatering stages before being used as acidifier in the precipitationstage. This usage of the hot flue gases as means for dewatering thelignin cake or lignin product could in one or several positions of theLignoboost process be implemented in parallel or preferably in series bysending the flue gases countercurrent to the lignin flow through theprocess.

In a further preferred embodiment of the inventive method is the entireprocess, from the second acidification stage, i.e. excluding the firstprecipitation stage which is kept alkaline, and until obtaining thefinal lignin product, kept at acidic conditions below pH 6. Preferablythe entire process from the second acidification stage is kept at acidicconditions even below pH 4. The pH level throughout the process is mostpreferred at a pH from 1 to 3.5. This would prevent any separated ligninfrom being dissolved again, and the precipitated lignin would besubjected to repeat leaching of metals and other unwanted components,meeting the objectives of obtaining a clean lignin product at highyield.

The inventive method may also include the additional steps of combiningthe pH level adjustment with an adjustment of the ion strength,preferably by using alkali metal ions or alkaline earth metal ions, mostpreferred calcium ions.

It is intended throughout the present description that the expression“dewatering” embraces any means of dewatering. Preferably the dewateringis performed by using centrifugation, a filter press apparatus, a bandfilter, a rotary filter, such as a drum filter, or a sedimentation tank,or similar equipment, most preferred a filter press apparatus is used.

It is intended throughout the present description that the expression“original black liquor” embraces spent cooking liquor from a digester,having most of the lignin from the original cellulose material dissolvedin the “original black liquor”. The “original black liquor” may alsohave a large content of organic and inorganic material, but may alsohave passed through separation processes for extracting turpentine orother specific constituents, while keeping the bulk volume of dissolvedlignin unaltered.

It is intended throughout the present description that the expression“lime kiln” embraces the conversion plant in the recovery island wherethe calcium carbonate in the lime mud obtained in the recaustizisingplant is calcined to calcium oxide and reused in the lime cycle.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 shows the prior art lignin separation process according to WO2006/031175.

FIG. 2 shows usage of lime kiln gases in the precipitation stage.

FIG. 3 shows usage of lime kiln gases in the precipitation stage as wellas using at least a part of the lime kiln gases for dewatering thelignin cake/product;

FIG. 4 shows usage of lime kiln gases in parallel in dewatering stages;

FIG. 5 shows usage of flue gases from lime kiln in series in severaldewatering stages.

FIG. 6 shows a process chart of one example of implementation of theinventive precipitation process using two vessels for the differentphases of the precipitation stage and a storage vessel in-between;

FIG. 7 shows an alternative implementation of the inventive method usinga single vessel for several phases of the precipitation stage,

FIG. 8 show an alternative implementation of the inventive method usingthree vessels for several phases of the precipitation stage,

FIG. 9 show results as of filterability of precipitated lignin usingdifferent process conditions.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is the known prior art process according to WO 2006/031175shown. The separation of lignin from original black liquor BL comprisingthe following stages in sequence:

-   -   a) Precipitation of lignin by a first acidification stage of the        original black liquor BL_(IN) by adding a first acid or mixture        of acids AC₁, in any suitable precipitation reactor PR,    -   b) followed by dewatering while forming a first filter cake with        high content of lignin, said dewatering made in any suitable        filter press FP₁, which may drain a first filtrate FL₁ from the        lignin suspension and have addition of gas blow trough G₁ of the        lignin cake in order to displace any residual acidic liquor,    -   c) suspending the first lignin filter cake obtained in stage b        in a second acidification stage using a second acid or mixture        of acids AC₂, said suspension made in any suitable reslurry tank        RT while discarding the odorous gases H₂S emitted,    -   d) whereupon a second lignin suspension is obtained in the        reslurry tank

RT,

-   -   e) dewatering of the second lignin suspension forming a second        filter/lignin cake with high content of lignin, said dewatering        made in any suitable filter press FP₂, which may drain a second        filtrate FL₂ from the lignin suspension, and at least a portion        of this second filtrate FL₂ may be re-circulated back to stage        c,    -   f) washing the second filter cake, said washing made in any        suitable wash apparatus WP, adding a wash liquid WL to this        washing stage, and finally    -   g) dewatering of the washed second lignin cake obtaining a        lignin product LP, said dewatering preferably made in the last        stages of the wash apparatus WP, which may drain a third        filtrate FL₃ from the second filter/lignin cake, and at least a        portion of this second filtrate FL₂ may be re-circulated back to        stage c, and may also have addition of gas blow trough G₂ of the        lignin cake in order to displace any residual acidic liquor.

In FIG. 2 are the basic steps of the precipitation process according toWO 2006/031175 shown. In this figure are flue gases G_(1a) obtained froma lime kiln LK sent directly to the precipitation stage PR. According topreferred embodiments of the invention should at least a part of thefirst acid or mixture of acids added to the first precipitation stage begases rich in carbon dioxide and having its origin from flue gasesvented from a lime kiln, meaning that the flue gases could be sentdirectly or indirectly to the precipitation stage.

In FIG. 3 is shown a further preferred embodiment the precipitationprocess according to WO 2006/031175. Here are at least a part of theflue gases G_(1a) vented from a lime kiln LK first used for dewateringthe lignin cake before being used as acidifier in the firstprecipitation stage, and the displaced residual gases G_(1b) is alsoadded to the precipitation stage PR together with lime kiln gases G_(1a)sent directly to the precipitation stage.

In FIG. 4 is shown a further preferred embodiment of the precipitationprocess according to WO 2006/031175. As disclosed earlier is the firstprecipitation stage PR and first dewatering stage FP₁ is followed by asuspension stage RT wherein the first lignin filter cake is suspended ina second acidification stage using a second acid or mixture of acidsAC₂, whereupon a second lignin suspension is obtained. This stage isthereafter followed by a second dewatering stage FP₂ of the secondlignin suspension forming a second filter cake with high content oflignin. A washing stage WP follows for washing the second filter cakeand finally followed by a third dewatering stage of the washed secondlignin cake obtaining a lignin product LP. According to the preferredembodiments of the inventive method are also the waste gases H₂S & CO₂emitted from the second acidification stage RT re circulated and mixedwith the original black liquor in the first precipitation stage PR. Inthis embodiment are the lime kiln gases sent directly and in parallelflows G_(1a) and G_(2a) to the dewatering stages FP₁ and WP, and thedisplaced residual gases G_(1b) and G_(2b) from these dewatering stagesare collected and added to the precipitation stage PR. Here are no fluegases from the lime kiln sent directly to the precipitation stage, butrather via said dewatering stages.

In FIG. 5 is shown an alternative embodiment of FIG. 4. In thisembodiment is the lime kiln gases sent directly to the last dewateringstage WP, and the displaced residual gases G_(2b) from this lastdewatering stage are collected and added to a preceding dewateringstage, here FP₁. The displaced residual gases G_(1b) from this precedingdewatering stage are collected and added to precipitation stage PR. Hereare no flue gases from the lime kiln sent directly to the precipitationstage, but rather via usage in said dewatering stages and countercurrentto flow of lignin trough the process.

As could be understood from these examples of embodiments could directfeed and/or indirect feed to precipitation stage via dewatering stagesof lime kiln flue gases be implemented in any possible ratio.

An additional procedure for stabilizing the lignin during the 2-stageprocess is, in combination with a pH-decrease, to adjust the ionicstrength in the slurry stage, preferably with multivalent alkali metalions or alkaline earth metal ions (e.g. calcium). At a given pH, ahigher ionic strength in the suspension stage reduces the lignin yieldlosses. Here would also the ionic strength and pH of the wash wateressentially corresponds to the conditions in the slurry stage to avoidgradients during the washing process. A higher ionic strength in theslurry and in the wash water gives a stable lignin and high lignin yieldeven at pH-values in the upper acidic range.

EXAMPLE

In FIG. 6 are shown typical process conditions for a two phaseprecipitation stage.

The actual example is using original black liquor from a kraft pulpingprocess for softwood having a pH level of 13.1 and a dry matter contentof 42%, and the figures may differ when using other black liquors.

Here are shown 2 carbonizing towers or vessels, PR1 and PR2, connectedin series and with a storage vessel ST in-between. For the understandingof the flow through the towers are open valves V₁, V₂, V₄ white andclosed valves V₃, V₅, black-filled. The chemical content of each flow isindicated in boxes either as total concentration in molecularconcentration M or in percentage.

The original black liquor BL_(IN) is fed to the top of the first towerPR1 via open valve V₁, and flows downwards to liquid pump LP1.

The carbonizing towers, PR1 and PR2 are preferably of differing designas to interior flow paths. The towers could be of simple elongatedvertical design with a square section. The first carbonizing tower PR1fed with original black liquor BL_(IN) is optimized for maximum contactarea between the black liquor and added acidifying gas and may contain arandom packing of filling bodies, preferably of a type like Rachig-ringsor other shapes of irregular filling bodies, said filling bodiespreferably having no dimension larger than 5 centimeter.

The filling bodies are so selected and installed in said tower such thatat least a part of the flow path of the first acidifying gas led troughthe first precipitation phase has a random flow path constantly changingflow direction at no straight flow path longer than 5 centimeter,preferably less than 1 centimeter, said flow path created by randompacking of filling bodies in said flow path.

After the first carbonizing tower PR1 is the acidified black liquorpumped to a storage vessel ST wherein the acidified black liquor isallowed to mature for at least 25 minutes.

The second carbonizing tower PR2 fed with acidified original blackliquor from the storage tower is optimized for avoiding blockage fromany precipitated lignin particles. If an undulated and extended gas flowpath is sought for in order to increase contact time between gas andblack liquor and hence increase the dissolving capacity of acidifyinggas could simple inclined lamellas be installed.

These lamellas are introduced to slow down the ascending motion of gastrough the tower and increase contact time between gas and liquid phasein order to dissolve most of the carbon dioxide. The inclination oflamellas should enable precipitate to fall downwards towards outlet andavoid accumulation of precipitate.

The inclined lamellas are so selected and installed in said tower suchthat at least a part of the flow path of the original black liquor fromthe first precipitation phase led trough the second precipitation phasehas an open flow path allowing a straight flow path longer than 5centimeter, with flow restrictions allowing precipitated lignin to movewith the flow of the black liquor with a flow deflection of theprecipitated lignin being less than 80 degrees in relation to thegeneral flow direction of the black liquor trough the secondprecipitation phase, hence allowing any precipitated lignin particlesflow with at least one flow vector being parallel to the general flow.

For handling some 100 m³/h of original black liquor could the height ofthe tower be some 8-10 meters, and the square section have a dimensionof 1.4×1.4 meter for the first tower PR1 and 1×1 meter for the secondtower PR2.

The lime kiln gases G_(1a) (corresponding to FIG. 3) are added to thebottom of the first tower PR1 via a flue gas pump GF, and any residualgases EG may be vented to atmosphere. As shown here is a large part ofthe carbon dioxide content in the flue gases dissolved in the firsttower PR1, from 29.7% down to 9.7%. The pH of the original black liquoris also lowered as a consequence from pH 13.1 down to 11.5. A firstsmall fraction of lignin is thus precipitated in this first phase of theprecipitation stage in the first tower as the amount of lignin in liquidform, LOH (aq), drops from 1.03 M down to 1.00 M, i.e. only less than 3%of the total lignin content. This small part of lignin precipitate onlycontains small lignin nucleus particles that are less prone to block thefilling of the first tower PR1.

After this first phase is the black liquor, now at pH 11.5, fed to thestorage tower ST.

In this storage tower the acidified black liquor is allowed to mature.

After the storage tower is the black liquor fed to the top of the secondtower PR2 via open valve V₂, and flows downwards before being fed outfrom the second precipitation phase via liquid pump LP2.

Lime kiln flue gases G_(1b) (corresponding to FIG. 3) having passed adewatering stage are added to the bottom of the second tower PR2, andany residual gases RG may be sent for combustion in a boiler, preferablythe recovery boiler. As shown here is a large part of the carbon dioxidecontent in the flue gases dissolved in the second tower PR2 as the pH ofthe original black liquor is further lowered to pH 11.2. A second largerfraction of lignin is thus additionally precipitated as well in thissecond phase of the precipitation stage in the second tower as theamount of lignin in liquid form, LOH (aq), drops from 1.00 M down to0.52 M, i.e. in total a precipitation in this phase of about 48%dissolved lignin fed to this stage.

In total it was found with these conditions that as much as 9.6 ton oflignin per hour was precipitated in these 2 phases, from a flow oforiginal black liquor in the order of 103 m³/h at a concentration of42%.

In FIG. 7 is an alternative embodiment with a single tower design forthe precipitation stage. Here is shown 4 phases Z₁/Z₂/Z_(ST)/Z₃ in saidtower having differing packing with filling bodies and lamellas. Here isthe original black liquor BL_(IN) fed in to the top of the tower andreaches a first phase Z₁ filled with small size filling bodies.Acidifying gas is added below this first phase via G_(1a) and flowsupwardly against the descending flow of black liquor. Residual gas isvented via EG.

Thereafter is the partly acidified black liquor from the first phasedescending down trough a second phase Z₂ filled with small size fillingbodies preferably with a larger size than those filling bodies of thefirst phase. Acidifying gas is added also below this second phase viaG_(1a) and flows upwardly against the descending flow of black liquor.

After the second phase is the further acidified black liquor from thesecond phase descending down trough a third storage phase Z_(ST). Herethe acidified black liquor is allowed to mature.

After the third storage phase is the acidified black liquor from thethird storage phase descending down trough a fourth phase Z₃.

The pH level at lower part of the second phase is preferably monitoredby at least a pH measurement as indicated to enable control of that thepH is close to condition for lignin particle precipitation of anysignificant order (preferably no more than small percentage at thisposition).

The actual pH level where lignin precipitation reaches higher valuesthan 2-5% may differ from the pH level identified in test, but ingeneral this level is typically about pH 11.5.

In this fourth phase Z₃ (similar to the second phase in FIG. 6) are thelamellas installed such that flow of the acidified black liquor is ledtrough the last phase has an open flow path allowing a straight flowpath longer than 5 centimeter. Preferably the flow restrictions, i.e.lamellas L, allow precipitated lignin to move with the flow of the blackliquor. As indicate with the arrow FD is indicated the general flowdirection trough this last phase, and the deflection angle α ispreferably less than 80 degrees in relation to the general flowdirection FD of the black liquor trough the second precipitation phase.This deflection allows any precipitated lignin particles to be flushedout and flow with at least one flow vector being parallel to the generalflow FD. If the deflection angle is 90 degrees could a stagnant zone becreated and lignin particles may start to accumulate.

In FIG. 8 is yet an alternative embodiment with a three tower design forthe precipitation stage. Here is shown a first acidification phase in anup flow tower, followed by a storage phase in a storage tower, andfinally a third acidification phase in a down flow tower, with densepacking with filling bodies in the first up flow tower.

In FIG. 9 are shown differing results as of filterability of theprecipitated lignin using different acidification modes. This plot showsthe amount of dry solids material, i.e. lignin precipitate, in kg persquare meter filter area, on the y-axis as a function of filtration timeon the x-axis.

-   -   Curve #1 (Filt 1) shows results from a process where the lignin        is precipitated by lowering the pH down to pH 10 without any        intermediate storage during the acidification. It indicates that        the filterability is fairly low needing a filtration time of        about 2000 seconds in order to obtain 8 kg of precipitate per        square meter filter area.    -   Curve #2 (Filt 2) show results from a two step acidification,        first to pH 10.7 and then finally to pH 10 with an intermediate        storage time of 60 minutes. It indicates that the filterability        is improved considerably only needing a filtration time of about        1000 seconds in order to obtain 8 kg of precipitate per square        meter filter area.    -   Curve #9 (Filt 9) show results from a two step acidification,        first to pH 11.5 and then finally to pH 10 with an intermediate        storage time of 60 minutes. It indicates that the filterability        is improved considerably only needing a filtration time of about        1000 seconds in order to obtain 8 kg of precipitate per square        meter filter area.    -   Curve #5 (Filt 5) show results from a two step acidification,        first to pH 11.25 and then finally to pH 10 with an intermediate        storage time of 60 minutes. It indicates that the filterability        is improved even further as compared to curves #2 and #9, only        needing a filtration time of about 500 seconds in order to        obtain 8 kg of precipitate per square meter filter area.

The results from tests shown in FIG. 9 indicate a potential for improvedfilterability when using a 2-stage acidification of the ligninprecipitation with an intermediate storage between the acidificationstages. For the specific black liquor tested, having a starting pH ofabout 13.1 in all examples, it shows that the filterability could beimproved almost 100% (by reducing filtration time from 2000 to 1000seconds) by using an intermediate storage after acidification to a pH ofabout 11.5 or 10.7 in a first acidification stage. There is also apossibility to find an optimum point of filterability, as shown in curve#5, showing that the filterability could be improved further almost 100%as compared with curves #2 and 9 (by reducing filtration time from 1000to 500 seconds) by using an intermediate storage after acidification toa pH of about 11.25 in a first acidification stage.

It is however likely that the absolute optimal point in pH after thefirst acidification stage may change depending on the specific blackliquor, i.e. may change between different pulp mills.

The reason for improved filterability is likely due to a phenomenonidentified as Ostwald ripening, that is allowed to occur during thestorage time. Ostwald ripening is a phenomenon where, over time,precipitated small lignin particles dissolve and redeposit onto largerprecipitated lignin particles. This redeposition onto larger ligninparticles occurs because larger particles are more energetically favoredthan smaller particles. While keeping the pH fairly constant during thestorage time would no new precipitation of small ligninparticles/nucleus be induced due to further reduction of pH, and thelignin particle growth of the initially precipitated lignin is insteadfavored.

Any further precipitation of lignin in the following secondacidification phase then predominately take place as lignin particlegrowth of the particles formed during the storage.

As the Ostwald ripening effect is a change of an inhomogeneous structureover time, should the storage time be implemented so that most of thiseffect could come into effect and the storage time should be at least 25minutes. Any increase in storage time above this minimum time is likelyto increase the particle size and thus improve the filterability of thefinal lignin particles. However, tests have shown that most part of thiseffect has been obtained after 45 minutes and only marginal supplementaleffects has been shown by extending the storage time more than 60 to 90minutes. As there is an investment cost associated with any storagevessel which increase with size is a storage time of about 60 minutes acommercially sound selection. It is to be noted that if the filtrationcapacity is improved such that the filtration time is reduced by 50%could much smaller separation equipment be selected.

If a reference filter is used for the separation process after atreatment according to curve #1, then a filter with only about 50% ofthis reference filter area would be necessary after treatment accordingto curves #2 and #9. And if the optimal treatment according to curve #5is used, then only about 25% of this reference filter area would benecessary.

It is to be noted that only a part of the lignin content is sought forprecipitation, as the residual black liquor BL_(OUT) is sent to theconventional recovery process, and thus a certain amount of lignin isneeded in order to maintain some of the combustible content, i.e. heatvalue, for the recovery boiler. Thus, it is of importance that theresidual black liquor after the precipitation process still is alkalineand do not add problems in the subsequent recovery process. TheLignoboost process is thus ideal for overloaded mills where the recoveryoperations in the evaporation plant or in the recovery boiler hasreached its operational limit, and further capacity for handlingincreased black liquor volumes is needed. Instead could the capacity ofthe pulping process be increased, and the increased black liquor volumesare met with a complementary process producing a “green” fuel of greatvalue.

While the present invention has been described in accordance withpreferred compositions and embodiments, it is to be understood thatcertain substitutions and alterations may be made thereto withoutdeparting from the spirit and scope of the following claims.

We claim:
 1. A method for separation of lignin from original blackliquor (BL_(IN)) having a first pH value, comprising the followingphases in sequence: a first precipitation phase (PR 1) wherein a firstacidifier charge is added to the original black liquor in order todecrease the pH value of the original black liquor to a second pH levelwhereby less than 10% of the total lignin content is precipitated asnucleus particles, said second pH level being at least 1 pH unit belowthat of the first pH value, a first storage phase (ST) wherein theoriginal black liquor is kept at or below the second pH level for aretention time of at least 25 minutes during which storage phase theprecipitated lignin particles increase in size thus increasing thefilterability of the precipitated lignin, a second precipitation phase(PR 2) wherein a second acidifier charge is added to the acidifiedoriginal black liquor from the first precipitation phase in order todecrease the pH value to a third pH level whereby more than 20% of thetotal lignin content is additionally precipitated and as growth ofnucleus lignin particles formed after the first storage phase, saidthird pH level being at least 1 pH units below that of the second pHvalue, and followed by a separation phase wherein the precipitatedlignin is separated from the remaining liquid phase of the acidifiedoriginal black liquor.
 2. A method according to claim 1 wherein theretention time in the storage phase is at least 45 minutes.
 3. A methodaccording to claim 2 wherein 50-80% of the total lignin content inoriginal black liquor (BL_(IN)) is precipitated in total after thesecond precipitation phase, and that the pH level of the acidifiedoriginal black liquor is still alkaline and has a pH level above 7.0after the second precipitation phase.
 4. A method according to claim 1wherein at least one of the first or second acidifier charge comprisesacidifying gas.
 5. A method according to claim 4 wherein the acidifyinggas is rich in carbon dioxide.
 6. A method according to claim 4 whereinthe acidifying gas has its origin from flue gases vented from a limekiln (LK).
 7. A method according to claim 4 wherein at least a part ofthe flow path of the first acidifying gas (G_(1a)) led through the firstprecipitation phase has a random flow path constantly changing flowdirection at no straight flow path longer than 5 centimeter, said flowpath created by random packing of filling bodies in said flow path.
 8. Amethod according to claim 4 wherein at least a part of the flow path ofthe original black liquor from the first precipitation phase led throughthe second precipitation phase has an open flow path allowing a straightflow path longer than 5 centimeter, with flow restrictions allowingprecipitated lignin to move with the flow of the black liquor with aflow deflection of the precipitated lignin being less than 80 degrees inrelation to the general flow direction of the black liquor through thesecond precipitation phase, allowing any precipitated lignin particlesflow with at least one flow vector being parallel to the general flow.9. A method according to claim 7 wherein the original black liquor(BL_(IN)) is flowing downwards in the first precipitation phase (PR 1)wherein a first acidifier gas is led countercurrent to flow of originalblack liquor.
 10. A method according to claim 7 wherein the originalblack liquor (BL_(IN)) is flowing upwards in the first precipitationphase (PR 1) wherein a first acidifier gas is led concurrent with flowof original black liquor.